The objective of this project is to use the techniques of somatic cell genetics to analyze the events leading to the gain of a cell surface antigenic determinant by a virus-transformed tumor cell. The antigens of the gp69/71 glycoprotein of Gross MuLV are being studied as a model system where gain occurs. The Thy-1 antigen in normal and virus-transformed BALB 3T3 cells is being studies as an example of a loss. Analysis of cell hybrids between cells expressing or not expressing a particular antigen is being used to define the genes involved in the control of antigen expression and to determine their mode of action.